The present application relates to a cathode active material, a cathode, a battery, a battery pack, an electronic apparatus, an electric vehicle, an electric storage apparatus, and an electric power system.
In recent years, along with widespread use of portable apparatuses such as video cameras, cellular phones, notebook personal computers, demand for small, lightweight and high-capacity secondary batteries as power supplies for the portable apparatuses are increasing. As a cathode active material for a secondary battery responding to such demand, in addition to lithium cobalt oxide (LiCoO2), lithium metal composite oxides such as lithium nickel oxide (LiNiO2) having the same layered structure with a R-3m space group as lithium cobalt oxide (LiCoO2) and lithium manganese oxide (LiMn2O4) having a normal spinel structure with a Rd3m space group have been put into practical use.
Whereas lithium cobalt oxide has a discharge capacity of about 150 mAh/g, lithium nickel oxide has a discharge capacity of about 180 mAh/g to about 200 mAh/g. Since nickel (Ni) as a raw material of lithium nickel oxide is lower in cost than cobalt (Co), lithium nickel oxide is superior also in cost to lithium cobalt oxide. Moreover, nickel is higher in raw material supply stability than cobalt. Therefore, lithium nickel oxide is superior also in raw material supply stability to nickel cobalt oxide.
On the other hand, a cathode active material using lithium nickel oxide has advantages of large theoretical capacity and high discharge potential; however, the crystal structure of lithium nickel oxide collapses with repetition of a charge-discharge cycle. As a result, a battery using lithium nickel oxide as a cathode active material has some issues such as a decline in discharge capacity and degradation in thermal stability.
To solve such issues, systems in which LiMn2O2 is mixed into nickel cobalt lithium manganese oxide or lithium nickel oxide with higher stability than lithium nickel oxide are widely used as cathode active materials. However, even though these systems are used as the cathode active materials, it is still necessary to improve cycle life performance and to suppress an increase in resistance during a cycle.
In Japanese Unexamined Patent Application Publication Nos. H08-213015, 2011-238416, and H09-298061, various proposals for lithium metal composite oxides are provided. For example, in Japanese Unexamined Patent Application Publication No. H08-213015, to improve self-discharge characteristics and cycle characteristics of a lithium secondary battery, there is proposed a lithium metal composite oxide represented by LixNiaCobMCO2, where 0.8≤x≤1.2, 0.01≤a≤0.99, 0.01≤b≤0.99, 0.01≤c≤0.3, 0.8≤a+b+c≤1.2 are satisfied, and M is one or more kinds of elements selected from a group configured of Al, V, Mn, Fe, Cu, and Zn.
In Japanese Unexamined Patent Application Publication No. 2011-238416, there is proposed a lithium nickel composite oxide with a layered rocksalt structure in which each of a lattice constant, an occupancy of lithium at a 3b site, and an occupancy of nickel at a 3a site obtained by a result of Rietveld analysis of a powder X-ray diffraction pattern is within a specific range. The lithium nickel composite oxide is obtained through finding a correlation between the occupancy at a lithium site (the 3a site) and the occupancy at a transition metal site (the 3b site) and the lattice constant obtained by the result of Rietveld analysis of the powder X-ray diffraction pattern, and discharge capacity and charge-discharge cycle characteristics. According to Japanese Unexamined Patent Application Publication No. 2011-238416, in the case where the lithium nickel composite oxide has a layered rock salt structure, and each of the lattice constant, the occupancy of lithium at the 3b site, and the occupancy of nickel at the 3a site obtained by the result of Rietveld analysis of the powder X-ray diffraction pattern is within a specific range, other sites are appropriately occupied by lithium and nickel to stabilize the crystal structure of the lithium nickel composite oxide; therefore, the lithium nickel composite oxide obtains stable and large discharge capacity and superior cycle characteristics as a cathode material.
In Japanese Unexamined Patent Application Publication No. H09-298061 there are proposed a cathode active material for nonaqueous electrolyte secondary battery which is allowed to be stably manufactured in an industrial-scale production process and has high initial discharge capacity and low resistance, and use of a spray dry method as a method of manufacturing the cathode active material. Japanese Unexamined Patent Application Publication No. H09-298061 proposes a cathode active material with high initial capacity in which a site occupancy of metal ions other than lithium at a 3a site and a site occupancy of metal ions other than nickel, cobalt, and manganese at a 3b site obtained by Rietveld analysis of a powder X-ray diffraction pattern are 5% or less and 10% or less, respectively, and an average particle diameter is 2 μm to 6 μm.